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AV receiver filter settings for use with subwoofer?

witwald

Active Member
I'm trying to get an understanding of how AV receivers with an LFE (Low Frequency Effects) channel need to be set up in order to work appropriately with a pair of full-range front speakers. The application is both music and videos. I think that I might have things correctly sorted out in my mind, but I'd like some confirmation of this.

Let's assume that I have a fairly newish AV receiver. Here are some scenarios with a variety of questions related to each one:

1) Assume that I don't have a subwoofer (heresy, I know:)). When viewing videos, what happens to the audio signal that would normally be sent to the LFE channel? Is it automatically sent to the front speakers? If so, is it split evenly between the left and right channels? I expect that the LFE channel would be rerouted to the front speakers, otherwise the viewer would miss out on all that audio information.

2) Assume that a subwoofer is present and connected to the LFE channel. When viewing videos, if the front speakers are set to "large" then can it be assumed that they will reproduce an unmodified signal, with the subwoofer reproducing only the LFE audio signal? What happens when this same setup is used to play standard audio CDs? In that situation, is the subwoofer given anything to reproduce? My feeling is that it isn't.

3) Assume that a subwoofer is present and connected to the LFE channel, and the front speakers are set to "small". When viewing videos, the front speakers have a highpass filtered signal sent to them. Is the complementary lowpass filtered signal from the left and right front speakers then sent to the LFE/subwoofer channel? This way, the front speakers no longer need to attempt to reproduce the deepest low frequencies, as the subwoofer just gets those components added to its LFE audio signal. Is that a correct picture of what's going on?
 
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Member 639844

Former Advertiser
1) All the LFE is sent out as a mono signal to both your front speakers when watching source material that includes a .1 channel.

2) This can change depending on the manufacturer. Some will play the .1 via the sub, as well as sending it to the speakers, but this is sometimes an option that has to be selected, usually called something like double bass. From what I understand, some receivers will play the front pair full range for all non .1 encoded material as well as sending info to the sub, and then .1 material is filtered as usual, but we need a few different AVR owners to have some input on that.

3) If the front speakers are set to small, then anything deemed 'large' in the content is sent to the sub, regardless of if the source has a .1 channel or not. Higher end receivers allow you to choose the frequency the speakers will and wont produce. Low end receivers usually just have a 'small' option and the cuttoff frequency of the speakers is then determined by the manufacturer. A lot of the time, this is all set by the receivers auto setup routine, and even most budget receivers include this these days.
 

HiFiRuss71

Distinguished Member
I'm trying to get an understanding of how AV receivers with an LFE (Low Frequency Effects) channel need to be set up in order to work appropriately with a pair of full-range front speakers. The application is both music and videos. I think that I might have things correctly sorted out in my mind, but I'd like some confirmation of this.

Let's assume that I have a fairly newish AV receiver. Here are some scenarios with a variety of questions related to each one:

1) Assume that I don't have a subwoofer (heresy, I know:)). When viewing videos, what happens to the audio signal that would normally be sent to the LFE channel? Is it automatically sent to the front speakers? If so, is it split evenly between the left and right channels? I expect that the LFE channel would be rerouted to the front speakers, otherwise the viewer would miss out on all that audio information.
That varies from manufacturer to manufacturer and amp to amp, but the full scale LFE signal is seldom split (indeed, some throw it away altogether) unmolested between the front channels. It is designed to reproduce the full 115dB Dolby/THX reference level peaks which is 10dB in excess of what the same standard allows to be sent to speakers. The results of this being allowed don't take much imagination and any receiver/processor that is found to be delivering un-neutered LFE to the main speakers should be avoided unless it's part of system specifically designed and setup to do so. That precludes everything up to and including the £250k system I experienced recently - All of the speakers were treated as 'Small'. True LFE requires a sub or the LFE will be at least dynamically compressed by properly designed bass management.

2) Assume that a subwoofer is present and connected to the LFE channel. When viewing videos, if the front speakers are set to "large" then can it be assumed that they will reproduce an unmodified signal, with the subwoofer reproducing only the LFE audio signal?
Correct.
witwald12430263 said:
What happens when this same setup is used to play standard audio CDs? In that situation, is the subwoofer given anything to reproduce? My feeling is that it isn't.
It isn't unless your receiver has an LFE+Main setting. This produces all manner of phase and room related issues that extend far further than the relatively restricted crossover a proper bass managed HP/LP crossover would and so is best avoided.

3) Assume that a subwoofer is present and connected to the LFE channel, and the front speakers are set to "small". When viewing videos, the front speakers have a highpass filtered signal sent to them. Is the complementary lowpass filtered signal from the left and right front speakers then sent to the LFE/subwoofer channel? This way, the front speakers no longer need to attempt to reproduce the deepest low frequencies, as the subwoofer just gets those components added to its LFE audio signal. Is that a correct picture of what's going on?
Exactly so. It's worth noting that it's easier (or more correct) to think of the subwoofer channel and an LFE soundtrack as two distinct issues. A subwoofer channel is simply something that handles bass from wherever it's coming, whilst the LFE soundtrack is discretely encoded audio channel (like FL, FR & C) that can only be reproduced full scale if a subwoofer is present.

Also worth noting is that most receivers/processors turn a standard 2 channel stereo signal into 2.1 under these same settings, whilst pressing the 'Pure Direct' button will, as well as turning off the video circuitry, also bypass the bass management and revert to a stereo 'Large' front pair. I had a Denon receiver, that in an unusually enlightened moment for a multinational, included a 'Direct' intermediate setting that bass manged the speakers, but still turned off the video side.

As an addendum, bass management can sometimes be a weird and wonderful concoction of what a particular manufacturer decides it to be. I can remember a well reviewed Sony (that was a good amp by all other accounts) from a couple of years ago that would redirect 'Small' surround speaker bass to the front stereo pair if a centre channel wasn't present, even if a sub was because the front speakers set to 'Large'. There are plenty of other instances of dynamic range compression being applied to all channels if a centre or surround channel was set to none, but a subwoofer still present too. You'd think it should be straight forward after over a decade of producing these things.....

Russell
 

witwald

Active Member
Moonfly and Russell,

Many thanks for the helpful explanations, which have made things much clearer for me. It seems that some care and attention need to be paid to the choice of receiver if good audio results are to be obtained with a subwoofer when playing audio CDs and videos. I'm now much better informed and as a result better able to exercise the required care.
 

Member 639844

Former Advertiser
Its a difficult one to get right with an AVR where music is concerned and all the manufacturers have different ideas about different things here and there. Demo'ing is always the way forward, but to dig deeper you will need to have a look at how their respective settings work in relation to music.
 

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